I am preparing the invitations for my in-laws' 50th anniversary reception
and am not sure if I should use the inner envelope (like a wedding
invitation) or just the gummed outer envelope.
Also, is it acceptable to print (from the computer) the return address on
the front of the envelope or on the back flap of the envelope, or must it be
handwritten (I am presuming that the addressee should be handwritten)?
Thank you!

I imagine how you address and stuff the invitation envelopes depends upon
how formal the event will be that you're hosting. There's no reason the
simplest combination can't be just as charming as the more complicated.
In addition, sometimes computer print-outs are much neater, and more
legible, than the handwritten addresses. Make up a couple example
options, then see which one you like best!

"X"

First, pat yourself on the back for taking the time and making the effort to honor your in-laws' successful marriage! With all the horrible things attributed to many in-law relationships, your caring is a wonderfully refreshing virtue! I applaud you!

Second, relax. No one will care how you address the invitations, just that they GET an invitation! Keep it simple and use just the gummed envelope with a nice clear computer font for the return address. I agree that handwritten would be best for the addressee, but if you haven't the best script - use a nice computer script font for that too. If done right, it'll look like it's been engraved.